diff --git a/src/html/helpdata.cpp b/src/html/helpdata.cpp
index 1864c907e5..82c9ec07d0 100644
--- a/src/html/helpdata.cpp
+++ b/src/html/helpdata.cpp
@@ -337,7 +337,12 @@ inline static wxChar* CacheReadString(wxInputStream *f)
 #endif
 }
 
-#define CURRENT_CACHED_BOOK_VERSION     3
+#define CURRENT_CACHED_BOOK_VERSION     4
+
+// Additional flags to detect incompatibilities of the runtime environment:
+#define CACHED_BOOK_FORMAT_FLAGS \
+                     (wxUSE_UNICODE << 0)
+
 
 bool wxHtmlHelpData::LoadCachedBook(wxHtmlBookRecord *book, wxInputStream *f)
 {
@@ -356,6 +361,9 @@ bool wxHtmlHelpData::LoadCachedBook(wxHtmlBookRecord *book, wxInputStream *f)
         return FALSE;
     }
 
+    if (CacheReadInt32(f) != CACHED_BOOK_FORMAT_FLAGS)
+        return FALSE;
+
     /* load contents : */
     st = m_ContentsCnt;
     m_ContentsCnt += CacheReadInt32(f);
@@ -393,6 +401,7 @@ bool wxHtmlHelpData::SaveCachedBook(wxHtmlBookRecord *book, wxOutputStream *f)
 
     /* save header - version info : */
     CacheWriteInt32(f, CURRENT_CACHED_BOOK_VERSION);
+    CacheWriteInt32(f, CACHED_BOOK_FORMAT_FLAGS);
 
     /* save contents : */
     for (cnt = 0, i = 0; i < m_ContentsCnt; i++)